The present invention relates to the coating of substrates by means of physical sputtering effect, in which the transfer of kinetic energy from ions of glow discharge plasma, striking the cathode surface leads to ejection of cathode material from the cathode and to subsequent formation of coating onto the substrate.
More particularly, the invention relates to sputtering process carried out in a diode-type apparatus, where glow discharge is established in the working chamber in an atmosphere of ionizable fluid, maintained at reduced pressure between the cathode, constituting a target, and the anode and where cathode atoms, emitted by the bombardment of plasma ions move towards the substrate, mounted in the same chamber.
The present invention also refers to articles, provided with the coating, deposited by the sputtering method.
The occurrence of a metallic coating which sputtered on the glass walls of a discharge tube was observed in the last century; the explanation for this phenomena at the beginning of this century, is the ejection of cathode material by positive ions striking the cathode.
Nowadays the sputtering process, which basically can be attributed to the same phenomena, is used on an industrial scale for the deposition of different kinds of coatings on different substrates. A large variety of devices has been developed for this purpose.
A comprehensive review of the physical methods of discharge sputtering deposition as well of equipment configuration, used for this purpose can be found in the monograph xe2x80x9cThin film processesxe2x80x9d, edited by John L. Vossen and W. Kern, Academic Press, 1978.
The simplest system, which is employed for deposition of films by sputtering process utilizes the glow discharge between two electrodes (cathode and anode), established within the evacuated working chamber and is commonly referred to as a diode arrangement.
The cathode in these systems is usually planar and constitutes the target, connected to a negative voltage source, capable of supplying voltage of several kV (dc or ac), while the anode is grounded. The substrate holder is mounted within the chamber and faces the cathode.
A stream of gas (the most common sputtering gas is argon) is continuously introduced into the chamber and is evacuated therefrom so as to provide a medium in which a glow discharge plasma, consisting of a gas ions, can be maintained.
The applied negative voltage urges these ions to strike the cathode, while removing its atoms by momentum transfer. The flux of these atoms moves towards the substrate, usually situated in the vicinity of the glow discharge region and condense onto the substrate surface.
In some cases gases or mixtures of gases other than argon are fed into the chamber so as to cause the deposition of a compound, synthesized on the substrate due to interaction between the atoms, which are dislodged from the cathode plate, and the reactive gas species, which are present in the chamber atmosphere.
TiO2, synthesized by sputtering a Ti cathode in a reactive atmosphere of mixture of argon with oxygen, can be mentioned as an example of such a compound.
The main advantage, associated with the planar diode arrangement is its simplicity and possibility of applying rather high voltages to the cathode, resulting in ejection and sputtering of atoms with high energies in the range of 10-100 eV, which reach the-substrate fast and thereon form a coating layer, firmly adhering to the substrate surface.
On the other hand, the high energy of plasma particles reaching the substrate is inevitably associated with exposing of the substrate surface to a heat flux with high density (up to several watt per square centimeter) and heating the substrate up to rather high temperatures of 400-600 deg.C.
Such high temperatures might have number of undesirable consequences, e.g.,
preclude the use of sputtering deposition for coating substrates, made of materials, susceptible to these temperatures e.g. plastics,
cause warping in substrates with a large length-to-thickness ratio,
deteriorate adhesion between the coating and the substrate due to the development of excessive thermal stresses in the interface region, because of the difference in linear thermal expansion coefficients of the coating and the substrate,
promote undesirable chemical reactions at the substrate surface, which is exposed to plasma ions and electrons, and the subsequent formation of undesirable compounds.
The other disadvantage of the planar diode arrangement is associated with relatively high gas pressure, which should be kept within the diode source chamber, so as to maintain condition for self-sustained glow discharge. This pressure might affect dispelling of the stream of the target atoms (so-called collision scattering), moving towards the substrate, which in its turn reduces the sputtering rate and prevents the establishing of conditions for formation of homogeneous coating and might even cause deterioration of some properties of the coating.
Reducing of the pressure can, to some extent, decrease the above-mentioned associated negative effects; however, this pressure can not be kept less than a certain minimum, which is 20-100 millibars; otherwise, the density of ions required for sputtering of target atoms falls too rapidly and sputtering rate becomes too slow.
Typical sputtering conditions, employed in planar high voltage diode sources, e.g., for sputtering of Ni in Ar atmosphere, as described in the above monograph are: cathode-to-substrate separation 4,5 cm, voltage 3 kV, pressure 75 millibar (7,5 KPa), current density 1 mA/cm2.
It is known that in order to improve sputtering rates at low pressures the ionization efficiency of available ionizing electrons of the gas should be increased. This effect is provided in diode sources, known as magnetrons, in which a transverse magnetic field, normal to the electric field is applied to the target and is so configured that the ExB electron drift currents close on themselves.
The magnetron mode of operation is defined by magnetic focusing of the glow discharge, which results in the formation of an uniform plasma sheet over the cathode, disposed remotely from the substrate surface and thus preventing its excessive heating, seeing that the substrate is no longer subject to the plasma bombardment.
A typical diode source, operating in the magnetron mode is disclosed, e.g., in U.S. Pat. No. 4,006,073, and comprises a cathode target, made of the sputtering material, a substrate holder mounted opposite said target, at least one anode, the means for supplying an ionizable gas, the means for establishing an electric field between the cathode and anode, sufficient to sustain an electrical discharge between them through said gas and means for establishing a magnetic field to extend through the space surrounding said anode and cathode.
Despite the benefits inherent to the magnetron mode of operation, like relatively low temperatures at the substrate surface (in the range of 50-200 deg.C.) and elimination of collision scattering effect due to reduced pressure, this configuration nevertheless suffers from its intrinsic limitations.
Configuration of magnetic field, formed as a closed-on-itself loop, causes a nonlinear current characteristic of the glow discharge area and does not the allow applyication of voltage as high as in planar diode sources.
The typical magnetron-mode operating conditions for magnetron type diode sources, as listed in the above mentioned monograph are:
voltage 800 V, magnetic field 150 G, pressure 1 millibar (100 Pa), current density 20 mA/cm2.
The relatively low voltage, employed in magnetron sources is associated with reduced energy, submitted to atoms being ejected from the target and moving to the substrate with energies of several eV instead of several tenths of eV. Reduced energy of target material flux, reaching the substrate is associated with formation of less dense coating and poor adhesion to the substrate.
A further serious disadvantage of the magnetron configuration is associated with reduced service life of the target due to inhomogeneous dislodging of cathode material and formation of regions with deep cathode erosion (local erosion profile), where magnetic and electrical field are crossed. Besides shortening the service life these regions together with less eroded regions define the target topography, which prevent the possibility of achieving a homogenous flux of material ejected from the target surface. The thickness of the coating becomes nonuniform and depends on the disposition of the substrate with respect to target surface.
The object of the present invention is to provide a sputtering deposition method and apparatus for its implementation, as well article of manufacture, comprising substrate and coating, deposited by this method in said apparatus, in which the above mentioned drawbacks of planar diode and magnetron diode modes of operation are sufficiently reduced or overcome, without losing, however, the benefits associated with each of these configurations.
In particular, the first object of the present invention is to provide new, simple and improved sputtering deposition method and apparatus, in which plasma is shifted away from the substrate and the substrate surface is not exposed to heat flux with high density so as not to undergo heating over 200-300 deg.C.
The second object of the present invention is to provide a new and improved sputtering deposition method and apparatus, in which the pressure of gas, supplied to the chamber does not exceed 1-7 millibars (0.1-0.7) Pa, being sufficient, however, for maintaining a self-sustained glow discharge and deposition with high sputtering rate.
The third object of the present invention is to provide a sputtering deposition method and apparatus, in which relatively high voltage of several kV can be applied to the cathode, while employing a magnetic field for focusing and localization of the glow discharge area.
Another object of the present invention is to provide a sputtering deposition method and apparatus, in which formation of local erosion profile through the target surface is sufficiently reduced, so as to improve the target service life, to achieve homogeneous ejection of target material and formation of uniform coating on the substrate, irrespective of mutual disposition of the substrate and the target.
Still another object of the present invention is to provide a sputtering deposition method and apparatus, as well as articles of manufacture, consisting of substrate and a coating firmly adhered thereon and having improved structure properties.
Yet another object of the present invention is to provide a mold for the manufacture of molded optical articles, consisting of glass substrate and silica coating, deposited by sputtering deposition method and apparatus, in which said coating has improved density, chemical resistance and optical properties.
The above and other objects and advantages of the present invention can be achieved in accordance with the following combination of its essential features, referring to embodiments of the present invention as a method, apparatus for its implementation and article of manufacture, produced in the apparatus for carrying out the method.
The method of depositing a coating onto a substrate by means of a sputtering process wherein a glow discharge plasma is generated in diode sputtering source between a cathode, provided with outwardly facing surface, constituting a target, and an anode, and wherein establishing of said plasma results in ion bombardment of said target, followed by ejection of target material and its movement towards said substrate with subsequent formation of a coating, depositing onto said substrate, whereas said method comprises the following steps:
introducing a to-be-coated substrate, into the process chamber so as to expose the surface of said substrate to the flux of the sputtered target material,
establishing a magnetic field within said chamber,
applying to said cathode electrical power sufficient for establishing a glow discharge plasma,
establishing within said chamber an atmosphere of ionizable fluid, continuously fed therein and evacuated therefrom so as to establish a working pressure, sufficient for maintaining the glow discharge plasma, consisting of ions of said fluid,
maintaining between said anode and cathode, a self-sustained glow discharge accompanied by generation of plasma, consisting of ions, bombarding said target and emitting target material towards said substrate,
characterized in that, said magnetic field being varied so as to achieve topology of its magnetic lines, defined by magnetic strength vector arbitrary oriented with respect to said anode and defined by magnetic strength in the range of 10-100 kA/m within the region, situated adjacent to said anode and in the range of at least 8-10 kA/m adjacent to said target and within a region, having the configuration of a layer, extending above the outwardly facing target surface, said layer having a thickness in the range of 2-5 cm; and said magnetic field being varied so as to localize and shift said plasma substantially away from said substrate.
In accordance with one of the preferred embodiments of the method, electrical power is supplied by means of dc or ac supply, being capable of supplying to said diode source voltage in the range of 1-5 kV, said magnetic field being established by means of solenoids or permanent magnets.
According to a further preferred embodiment said ionizable fluid is continuously fed within said chamber so as to monitor therein a working pressure in concert with variations in electrical power and/or magnetic field.
As per still another preferred embodiment said ionizable fluid consists of an inert and/or reactive gas.
According to another preferred embodiment said working pressure of said ionizable fluid is maintained at less than 1 Pa, preferably in the range of 0.1-0.7 Pa.
In accordance with a further embodiment of the present invention, there is provided an apparatus suitable for depositing a coating onto the substrate by means of a diode sputtering source wherein a glow discharge plasma is generated between cathode, constituting a target, and anode, and wherein establishing of said plasma results in ion bombardment of said target, followed by liberation of target material and its ejection and transport towards said substrate with subsequent formation of a coating, depositing onto said substrate, said apparatus substantially consisting of:
a vacuum chamber containing an atmosphere of ionizable fluid fed within said chamber at a controlled reduced pressure
a cathode pole structure, positioned within said chamber and defined by an outwardly facing surface, constituting a target, composed of material to be ejected therefrom,
an anode, insulated electrically from said cathode pole structure and situated within said chamber so as to provide diode arrangement with said cathode pole structure,
an electric power supply means for applying electrical potential between said target and said anode, abd sufficient for establishing and maintaining a self sustained plasma glow discharge between them,
at least one substrate holding means, mounted within said chamber and positioned so as to expose said substrate to the flux of target material ejected therefrom,
a means for supplying ionizable fluid into said chamber and its evacuation therefrom so as to maintain within said chamber the atmosphere of said fluid at pressure sufficient for maintaining said plasma glow discharge,
a magnetic field generating means for establishing magnetic field within said chamber,
an appropriate control and instrumentation means for monitoring electrical and magnetic parameters of said power supply means and said magnetic field generating means,
characterized in that said magnetic field generating means is situated substantially adjacent to said anode and is capable of generating a magnetic field with topology defined by magnetic strength vector arbitrary oriented with respect to said anode and having magnetic strength gradient within the interior of said chamber, said magnetic strength preferably being in the range of 10-100 kA/m in the vicinity of said anode and in the range of at least 8-10 kA/m in the vicinity of said cathode within a region, extending above the target surface and having a thickness of 2-5 cm.
According to one of the preferred embodiments of the present invention, implementing it as an apparatus suitable for carrying out of said method, said electrical power supply means is a dc supply, being capable of supplying to said diode voltage in the range of 1-5 kV, said magnetic field generating means being formed as solenoids or permanent magnets.
In still another preferred embodiment said apparatus comprises a pressure control means adapted to monitor pressure within said chamber in accordance with variation of parameters of said electrical power supply and/or magnetic field generating means.
As per yet a further preferred embodiment, the material of said target consists of a metallic element and said ionizable fluid is a mixture of argon and oxygen.
According to still another preferred embodiment said outwardly facing target surface of said cathode pole structure is formed as a body, defined by central symmetry with respect to its center of symmetry; said anode is formed as a body, having rotational symmetry with respect to its central axis of symmetry; said anode surrounds said cathode pole structure; said substrate holding means is formed as a body, having central symmetry with respect to its center of symmetry; said substrate holding means is provided with a plurality of holding stations arranged thereon for fixation of a plurality of substrates; said cathode pole structure, said anode and said substrate holding means being arranged in such a manner, that both said centers of symmetry of said target surface and said substrate holding means and said central axis of symmetry of said anode substantially coincide.
In a further preferred embodiment said cathode pole structure is formed as a spherical segment and said substrate holding means being formed as a truncated icosahedron.
According to another preferred embodiment of the present invention it can be implemented in an article, manufactured within said apparatus by carrying out of said method, said article of manufacture consisting of a substrate and a coating deposited onto said substrate, whereas said coating is formed on the surface of said substrate due to interaction of material, sputtered from the cathode target with the ionized reactive fluid, fed into said source; wherein sputtering of said material towards said substrate is effected by glow discharge plasma magnetically shifted away from said substrate and said coating, deposited onto said substrate being formed at temperatures not exceeding 300 deg.C., preferably 40-200 deg.C.
In still a further preferred embodiment said coated article constitutes a mold for manufacturing of optical articles, said substrate consisting of glass and said coating consisting of SiO2 film.
The present invention in its various embodiments has only been summarized briefly.
For better understanding of the present invention as well of its advantages, reference will be now made to the following description of its embodiments, taken in combination with accompanying drawings, tables and examples.